Molecular shape molecules with multiple bonds

Molecules with multiple bonds between metal atoms often have structures with beautiful and highly symmetrical polyhedral shapes [3-52], The square prismatic [Re2Clg] ion [3-53], shown in Figure 3-36, played an important role in the history of the discovery of metal-metal multiple bonds. Figure 3-37a shows another molecular model with a metal-metal multiple bond. Its shape is similar to the paddles that propel riverboats. There is then a whole class of hydrocarbons called paddlanes [3-56], and one of their representatives is shown in Figure 3-37b. 

The shape of a molecular particle plays a major role in determining the macroscopic properties of a substance. We examine this role in other chapters in this book. To better understand and predict the shape-property relationship, you should know what is responsible for molecular shape, and this is discussed in this section and the next. Discussion in these sections is limited to molecules having only single bonds. Molecules with multiple bonds are considered in Section 13.4. 

Resorcin[4]arene and pyrogaUol[4]arene (Scheme 3.4) have the ability to easily form hydrogen bonding molecular capsules. They possess multiple hydroxyl groups at the upper rim, which can interact with each other or surrounding solvent molecules. Their bowl-shaped conformation is stabilized by four intramolecular O-H- -O hydrogen bonds. Four electron-rich aromatic rings can bind cationic and neutron... 

Two main approaches to the control of molecules using wave interference in quantum systems have been proposed and developed in different languages . The first approach (Tannor and Rice 1985 Tannor et al. 1986) uses pairs of ultrashort coherent pulses to manipulate quantum mechanical wave packets in excited electronic states of molecules. These laser pulses are shorter than the coherence lifetime and the inverse rate of the vibrational-energy redistribution in molecules. An ultrashort pulse excites vibrational wave packets, which evolve freely until the desired spacing of the excited molecular bond is reached at some specified instant of time on a subpicosecond timescale. The second approach is based on the wave properties of molecules as quantum systems and uses quantum interference between various photoexcitation pathways (Brumer and Shapiro 1986). Shaped laser pulses can be used to control this interference with a view to achieving the necessary final quantum state of the molecule. The probability of production of the necessary excited quantum state and the required final product depends, for example, on the phase difference between two CW lasers. Both these methods are based on the existence of multiple interfering pathways from the initial... 

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